Date: October 20, 2009
Title: Mercury: Close Encounter for the Third Time
Podcaster: Bob Hirshon
Organization: American Association for the Advancement of Science (AAAS). Find more information about MESSENGER mission
Description: On September 29th, the MESSENGER spacecraft made its third and final flyby over the surface of the planet Mercury, in preparation for orbital insertion in March of 2011. The mission is the first attempt to put a spacecraft in orbit around the planet. The planet’s small size and proximity to the Sun make this task incredibly difficult to achieve. Today’s podcast takes you live to the Johns Hopkins Applied Physics Laboratory, where the MESSENGER Engineering and Operations team follows the spacecraft as it makes its third flyby of Mercury.
Bio: Bob Hirshon is Senior Project Director at the American Association for the Advancement of Science (AAAS) and host of the daily radio show and podcast Science Update. Now in its 22nd year, Science Update is heard on over 300 commercial stations nationwide. Hirshon also heads up Kinetic City, including the Peabody Award winning children’s radio drama, McGraw-Hill book series and Codie Award winning website and education program. He oversees the Science NetLinks project for K-12 science teachers, part of the Verizon Foundation Thinkfinity partnership. Hirshon is a Computerworld/Smithsonian Hero for a New Millenium laureate.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Drew Roman.
Transcript:
Welcome to today’s edition of the 365 Days of Astronomy podcast. I’m Bob Hirshon, host of AAAS Science Update radio and its companion podcasts.
Today, I’m at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland with the engineering and operations team for the MESSENGER spacecraft mission to the planet Mercury. In a short while, MESSENGER will skim by the surface of Mercury for the third time in the past two years. At it’s closest point, it will be just 228 km from the surface.
MESSENGER is the first spacecraft to visit Mercury since Mariner 10 way back in 1975. Even though Mercury is quite close to the Earth, it’s extremely difficult to visit. That’s because it’s even closer to the Sun, so any spacecraft visiting Mercury has to contend with the Sun’s extreme heat and gravitational attraction.
To get into orbit around Mercury, MESSENGER has to go through all sorts of gyrations to adjust its angle and speed. It was launched over five years ago in August of 2004. A year later, it flew by Earth, using the planet’s gravity to sling it at the right angle and velocity to complete two loops by planet Venus in 2006 and 2007. That sent the craft into a path that would allow three flybys of Mercury itself, two of them last yera and one coming up in just a short while.
All that maneuvering has been necessary to slot MESSENGER into the proper path to enter a stable orbit around Mercury a year and a half from now, in March of 2011. But during the three fly-bys, MESSENGER doesn’t just sit tight. Its eight primary instruments and experiments swing into action, recording detailed measurements of the planet.
The last two fly-bys already produced surface images of most of the planet, including the more than half of the surface that Mariner 10 missed. They also sent back measurements of the planet’s magnetic field, its gravity, and other readings that help scientists understand Mercury’s composition and origin.
(About 45 minutes later)
Okay, and I have with me Jim McAdams, and what’s your title?
McAdams: I’m Mission Design Lead Engineer for MESSENGER.
Hirshon: And we’re about ten minutes away. I think a lot of people, lay people especially, would have this image of MESSENGER sort of zooming along like a flying video camera, but what is it actually doing right now?
McAdams: Right now it’s approaching Mercury at a speed of over 5 km per second or about 3 miles per second, and they’re preparing to do more imaging as they pass the close approach of Mercury. Just after close approach, they’re going to be doing a lot of surface scans with instruments that don’t image the surface but they do minerology and other composition type data.
Hirshon: And in the other room I was looking at a video of it; it showed the actual spacecraft kind of moving around and doing all these gyrations—it was almost like a ballet. So what’s going on?
McAdams: Well, every single attitude is precisely planned and this has been run through, tests and simulations, multiple times until they’ve got it just right. They have to make sure that the sun shade is pointed toward the sun during all this and the solar panels are tilted so they don’t over heat. And then there’s all the various orientations the spacecraft has to achieve for the science measurements.
Hirshon: And does the whole craft have to turn to make those things point the way they need to point?
McAdams: Basically it keeps the large sun shades toward the sun and then it rolls back and forth as the instruments have to point in different directions as it’s looking behind Mercury and then it turns and points the cameras at Mercury and the cameras themselves can pivot to a range of about 90 degrees towards or away from the sun. So there’s a lot of activity and thousands of movements during this fly by time.
Hirshon: And how far is it from the planet now?
McAdams: Right now it is 837 km altitude and going down to a minimum of 228…
Hirshon: Wow, incredibly close. Okay, looks like we’re about seven minutes away.
(Two minutes later)
Hirshon: Okay, a lot of the science team is coming back right now as we approach… less than five minutes to closest approach to Mercury.
(Three minutes later)
Hirshon: We’re at about two minutes…
(One minute later)
Hirshon: Exactly one minute to closest approach.
McAdams: We’re at less than one minute!
Hirshon: All right… and what’s that yellow line? Is that the altimeter?
McAdams: That’s the footprint of where the laser is hitting the surface. And it’s moving across the surface pretty rapidly because the spacecraft is moving pretty rapidly at close approach.
Hirshon: Thirty seconds
McAdams: One second. So now we’re at close approach.
Voice off mic: Did you feel it?
McAdams: I felt a breeze!
Voice off mic: It’s an emotional thing…
McAdams: Right, right. I’d high five someone but they might get wet.
Hirshon: I don’t see people jumping up and down or sighing with relief.
McAdams: We’ve been through this before and you don’t lose contact and you just… you know it’s gonna work.
Hirshon: And now it’s already going away! It’s leaving so soon! So when do we get actual information and pictures and everything?
McAdams: That starts very late this evening. I believe between 11:30 and 12 o’clock midnight, local time here.
(About fifteen minutes later)
Voice on PA: For those following along, we do have an unexpected signal drop that we’re now troubleshooting. So stand by.
Hirshon: We have a slight hitch, a signal drop that they’re troubleshooting right now…
(A little later.)
Another voice on PA: MESSENGER RF would like you to call them on a black phone. The extension here is 0324…
(Later)
First PA Voice: This is MESSENGER Ops with a status update. We’re currently troubleshooting to see if there’s an RF configuration on board the spacecraft that may have dropped out the signal. We are coming up on a 51 minute blockage period with occultation. And when we come out on the backside there are some on board commands that switch the configuration so we’ll be looking for that next.
Hirshon: Earlier, when they mentioned the black phone—I don’t know if you heard that, calling on the “black phone”—they were using another ground station to try to pick up a signal. They have redundancy, so they have other ways to search for the signal and that came up negative, too, so it was not a ground based problem. It was actually that the spacecraft wasn’t transmitting apparently.
(40 minutes later)
Hirshon: So we have about ten minutes before Mercury comes out of occultation which is when the planet is actually in the way and we can’t get any signal from it. But just a few minutes before it went into occultation, we lost the doppler signal and it went dark. And the scientists huddled. They just came out of their meeting and they went through different scenarios that could explain it. And they came up with something to do with the orientation of the antennas—the radio antennas that send the signal back to Earth—and if they’re right, it’s going to be self-correcting and when the spacecraft comes out of occultation in a few minutes, it will have repaired itself, and we should be able to get a signal.
(A few minutes later, just before MESSENGER is scheduled to re-emerge from Mercury occultation)
Hirshon: Okay, it’s 23:00:19… woo-hoo! All right! Signal has been restored (runs from Mission Control room to conference room) The signal’s back!
Voice off mic: It’s back? Yay!
Voice on PA: This is MESSENGER Ops. We obviously have confirmation of the signal at the expected time, which confirms that the command load is still running nominally as expected. So we’re back to the normal time line and we’ll obviously have an investigation after we get housekeeping data several hours from now as to what actually occurred.
Hirshon (to MESSENGER Project Manager Peter Bedini): Did we find out whether we lost any data, or they don’t know yet?
Bedini: As far as we know, the only data that was lost was a small amount of radio science data that was being collected to help do measurement of the gravity of the planet. As far as the other science measurements, we have no reason to believe the sequence hasn’t continued as planned. So all the laser and other laser and imaging and stuff like that, as far as we can tell, was collected. We’ll know more when we get some housekeeping down around 9:30 or after 9:30 tonight and we’ll look into it and see. We’re suspecting that radio antenna—the RF antenna—did not switch as we had hoped it to do. But it doesn’t affect the course of the spacecraft or the science collection at all.
Hirshon: Peter Bedini, from the science team. So we’re back to all systems go.
(The next day, Septemer 30, 2009)
Hirshon: Okay, now it’s the next morning. It’s September 30th, and we’re back at the Applied Physics Laboratory. And we’re getting a report from the various team members on the results of their various trouble-shooting of the spacecraft last night.
The results weren’t what they had hoped. Actually, it wasn’t just a matter of the antenna not sending the signal and the spacecraft continuing to gather data. The spacecraft actually went into “safe mode.” The easiest way to explain it is it’s as if a circuit breaker was tripped when the solar panels switched off and the batteries switched on because the spacecraft was in eclipse behind planet. At that point, the fault protection tripped, went into safe mode, so the spacecraft is in good shape, there’s no actual malfunction of any of the instruments. But it was not operating from a few minutes before closest approach to well into its departure, when they were able to bring it back on line. So that information was not gathered and was lost.
The good news is that was terrain that had been covered on the first two fly bys and terrain that will be covered again when MESSENGER comes back in a year and a half and goes into orbit around Mercury.
So there was a lot of redundancy built into this mission. But the good news is that the spacecraft is in good condition, and the trajectory is just exactly what they want, so that’s what’s important.
And behind me is Eric Finnegan, he is the Mission Systems Engineer and he’s giving a run down, a much more technical explanation of what I was just saying. And what’s really cool is on the phone we have Bob Strom who is a science team member of MESSENGER, but he’s also the only scientist who was also a member of the original Mariner 10—that was 36 years ago. So it’s very exciting for him and he’s participating in this meeting by phone.
Eric Finnegan: … the same piece of Mercury at all of those phase angles and covers the same phase angle range that we’ll be using for orbital core mapping. So that’s a partial recovery. Next? Oh, that’s it.
Bob Strom: Can you tell me, I missed the 11 o’clock conference. How is the spacecraft?
Finnegan: The spacecraft is nominal and all of the subsystems are fully functional. There was an issue with, basically, an on-board alarm limit that was tripped. And it caused the spacecraft to “safe.” It wasn’t really so much of a hardware problem as that the alarm limits were set very tight.
Strom: Okay, so now everything is functioning normally.
Finnegan: Yes.
Strom: Oh, well that’s good news. I’m just sick that we lost all the high resolution coverage, but as long as the spacecraft is doing okay, that’s all I care about.
Finnegan: Well…
Strom: I damn near had a heart attack when I saw that message.
Finnegan: So, I’ll take the liberty of paraphrasing Sean (Sean Solomon, Principal Investigator for the MESSENGER mission): We’ll be back!
Strom: Good! Good.
Hirshon: And that’s it for today’s version of 365 Days of Astronomy. This is Bob Hirshon signing off from the Applied Physics Laboratory in Laurel, Maryland.
End of podcast:
365 Days of Astronomy
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